fix: 水印配置缺失时 xlsx 预览白页 —— officeweb.ftl 补上缺省值

未提供外部 config/application.properties 的部署上，预览任何 xlsx 都失败：
FreeMarker 在 officeweb.ftl 第 25 行抛 InvalidReferenceException，点名
watermarkTxt。DEBUG 模式把错误内联进 HTML，页面断在半截 JavaScript 里、
永远没有 </html>，浏览器上是白页或一直转圈，而 HTTP 状态码是 200 ——
看起来像转换超时，不像模板错。

两处叠在一起：

1. 十个 watermark* 请求属性可能整族不存在。WatermarkConfigConstants 没有
   类级注解（对比 ConfigConstants 上的 @Component），不是 Spring bean，
   那十个 @Value 的 setter 从来不会被调用；唯一填充它们的
   ConfigRefreshComponent#loadConfig() 在配置文件不存在时直接 return。
   于是十个 getter 全返回 null，AttributeSetFilter 的
   setAttribute(name, null) 按 Servlet 规范等同于删除属性。

2. officeweb.ftl 是唯一没有 classic_compatible 兜底的预览模板。
   commonHeader.ftl 第 1 行是 <#setting classic_compatible=true>，
   所有 include 它的模板在属性缺失时把变量渲染成空串、照常出页面；
   officeweb.ftl 既不 include 它、自己也没有这个设置。

缺省值取 WatermarkConfigConstants.DEFAULT_*，watermarkTxt 取空串——
空串会让下一行的 if (watermarkTxt !== '') 跳过 watermark.init()，不会给
所有预览凭空加上水印。其余九个只在水印开着时才用到，但那个 if 是
JavaScript 的、不是模板的，FreeMarker 两条分支都渲染，所以十个都要给。

新增的测试直接用 FreeMarker 渲染这个模板，不需要把服务跑起来：
数据模型里一个 watermark* 都不给（那正是缺陷发生时的情形），断言渲染得出
</html>；另一条把十个值全给上，断言逐个原样渲染，保证缺省值不会盖掉真实配置。
改模板之前先跑过一遍，第一条报 InvalidReferenceException、第二条通过。

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

server/src/test/java/cn/keking/web/OfficeWebWatermarkDefaultsTests.java

@@ -0,0 +1,92 @@
+package cn.keking.web;
+
+import freemarker.template.Configuration;
+import freemarker.template.Template;
+import freemarker.template.TemplateExceptionHandler;
+import org.junit.jupiter.api.Test;
+
+import java.io.StringWriter;
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * officeweb.ftl is the only preview template that neither includes commonHeader.ftl nor
+ * turns on classic_compatible, so it is the only one that fails outright when the
+ * watermark request attributes are absent.
+ *
+ * They are absent whenever WatermarkConfigConstants has not been populated: it is not a
+ * Spring bean, so its @Value defaults never run, and ConfigRefreshComponent#loadConfig
+ * returns early when the external config/application.properties is missing. Every getter
+ * then returns null, and AttributeSetFilter's setAttribute(name, null) removes the
+ * attribute per the Servlet spec.
+ *
+ * The first test therefore renders with a model that carries no watermark* entry at all.
+ * Do not add them "to make it realistic" - that is the condition under test.
+ */
+class OfficeWebWatermarkDefaultsTests {
+
+    @Test
+    void shouldRenderXlsxPreviewPageWhenWatermarkAttributesAreMissing() throws Exception {
+        String html = render(baseModel());
+
+        assertTrue(html.contains("</html>"), "page was truncated before </html>");
+        assertTrue(html.contains("let watermarkTxt = '';"), "watermarkTxt did not fall back to an empty string");
+        assertTrue(html.contains("watermark_width: 240,"), "watermarkWidth did not fall back to its declared default");
+        assertTrue(html.contains("watermark_font: '\u5fae\u8f6f\u96c5\u9ed1',"), "watermarkFont did not fall back to its declared default");
+    }
+
+    @Test
+    void shouldKeepConfiguredWatermarkSettingsWhenAttributesArePresent() throws Exception {
+        Map<String, Object> model = baseModel();
+        model.put("watermarkTxt", "kkFileView");
+        model.put("watermarkXSpace", "20");
+        model.put("watermarkYSpace", "30");
+        model.put("watermarkFont", "Arial");
+        model.put("watermarkFontsize", "24px");
+        model.put("watermarkColor", "red");
+        model.put("watermarkAlpha", "0.5");
+        model.put("watermarkWidth", "300");
+        model.put("watermarkHeight", "120");
+        model.put("watermarkAngle", "45");
+
+        String html = render(model);
+
+        assertTrue(html.contains("let watermarkTxt = 'kkFileView';"));
+        assertTrue(html.contains("watermark_x_space: 20,"));
+        assertTrue(html.contains("watermark_y_space: 30,"));
+        assertTrue(html.contains("watermark_font: 'Arial',"));
+        assertTrue(html.contains("watermark_fontsize: '24px',"));
+        assertTrue(html.contains("watermark_color: 'red',"));
+        assertTrue(html.contains("watermark_alpha: 0.5,"));
+        assertTrue(html.contains("watermark_width: 300,"));
+        assertTrue(html.contains("watermark_height: 120,"));
+        assertTrue(html.contains("watermark_angle: 45,"));
+    }
+
+    /** Everything officeweb.ftl needs except the watermark attributes. */
+    private Map<String, Object> baseModel() {
+        Map<String, Object> model = new HashMap<>();
+        model.put("file", Map.of("name", "demo.xlsx"));
+        model.put("pdfUrl", "demo.xlsx");
+        model.put("baseUrl", "http://127.0.0.1:8012/");
+        model.put("kkagent", "false");
+        model.put("kkkey", "");
+        model.put("xlsxshowtoolbar", Boolean.TRUE);
+        model.put("xlsxallowEdit", Boolean.FALSE);
+        return model;
+    }
+
+    private String render(Map<String, Object> model) throws Exception {
+        Configuration configuration = new Configuration(Configuration.VERSION_2_3_32);
+        configuration.setClassLoaderForTemplateLoading(getClass().getClassLoader(), "web");
+        configuration.setDefaultEncoding("UTF-8");
+        configuration.setTemplateExceptionHandler(TemplateExceptionHandler.RETHROW_HANDLER);
+
+        Template template = configuration.getTemplate("officeweb.ftl");
+        StringWriter out = new StringWriter();
+        template.process(model, out);
+        return out.toString();
+    }
+}